Several countries have tightened or reviewed airport security following twin explosions at Brussels airport.

The Islamic State of Iraq and the Levant (ISIL, also known as ISIS) claimed responsibility for attacks in the departure hall of Zaventem Airport, and a rush-hour metro train which killed at least 30 people.

Belgians paid tributes outside the Brussels Stock Exchange after attacks on Tuesday that claimed the lives of more than 30 people and injured dozens.

Others used chalk to write words of solidarity on the ground.

A woman consoles her children at a street memorial [Vincent Kessler/Reuters]

The prime minister called Tuesday a 'black day' for Belgium [Charles Platiau/Reuters]

The attacks come just days after a key suspect in the Paris attacks, Salah Abdeslam, was captured in Brussels [Carl Court/Getty Images]

People gather at the Place de la Bourse following the attacks [Carl Court/Getty Images]

Belgium raised its terror alert to its highest level, diverting arriving planes and trains and ordering people to stay where they were [Carl Court/Getty Images]

More than 30 people were killed after Brussels airport and a metro station were targeted by explosions [Carl Court/Getty Images]

Messages left by mourners, including a message that reads: ''No Forgiveness!'' and another one that reads: ''I''ll piss on your bombs'' at the Belgian embassy in Berlin [Sean Gallup/Getty Images]

Police patrol the EU commission building, after a bomb exploded at a metro station nearby [Martin Meissner/AP]

Activists of the Azov civil corp commemorate the victims near the Belgian embassy in Ukraine [Valentyn Ogirenko/Reuters]

German police officers guard a terminal of the airport in Frankfurt, Germany [Michael Probst/AP]

Two people write solidarity messages in chalk outside the stock exchange in Brussels [Geert Vanden Wijngaert/AP]

New York authorities are increasing security throughout the city following the blasts [Richard Drew/AP]

Police control access to the central train station in Brussels [Christian Hartmann/Reuters]